A two component polyurethane coating consisting of a hardener component of polyisocyanate compound and a resinous component of hydroxyl group containing film-forming resin is being watched with keen interest in various technical fields including auto-repair, wood processing and other industries, because there is no necessity for relying on a high temperature baking operation.
In such a coating, it is highly desired to be of fast-drying nature from the view-point of reduction in processing steps and energy consumption.
Various attempts have been, therefore, made to give the desired fast-drying characteristics to two component polyurethane coatings, but no satisfactory solutions have been found yet.
For example, in an attempt of utilizing as a resinous component an acrylic resin with a higher molecular weight or a higher glass transition temperature (Tg), there are such problems that since a spraying non-volatile matter is in a lower order, the coating must be applied many times and moreover, the coating thus obtained is, in general, no good in appearance.
In an attempt of compounding with a cellulosic resin as nitrocellulose, cellulose acetate butyrate (CAB) and the like, there are almost similar problems as abovementioned. When an increased amount of catalyst is used, the pot-life of the resulted composition is proportionally decreased. And the coating thus obtained is poor in appearance. In an attempt of using as a hardener an aromatic polyisocyanate compound, there is a problem of poor weather resistance. Thus, none of the aforesaid attempts have succeeded in having fruitful results.
Recently, the so-called gellated resin particles having three-dimensionally crosslinked structures and being substantially insoluble in an organic solvent have been proposed. If the gellated resin particles are in the two component polyurethane coating system, it would be probable that the desired fast-drying might be obtained therewith from the combination of pigment filler effect as plastic pigment and hard resin effect of the crosslinked polymer particles.
However, the heretofore proposed crosslinked polymer particles were merely developed for the purpose of high solidification or rheology control of a solvent type coating composition, thereby making up the drawbacks of sagging and cratering tendencies and of deficient weather resistance or the like. Even the slightest consideration had been made as to the possible use of these crosslinked polymer particles in the room temperature curing type polyurethane coatings. Therefore, even when examined with a two component polyurethane coating to be cured at a room temperature, it was unable to get the product being satisfied with every respects of application characteristics, drying property and dispersion stability. The coating thus obtained was in fact poor in gloss, transparency and finishing appearance. Under the circumstances, it is a principal object of the invention to provide a two component polyurethane coating containing crosslinked polymer particles, which is excellent in dispersion stability, application characteristics and drying fastness, capable of resulting a coating with excellent gloss, transparency and appearance, as well as excellent polishing resistance, and which is particularly useful as an auto-repair paint and the like.